Intraluminal medical devices, such as stents, can be implanted into a body lumen during a non-invasive clinical procedure, to reinforce, support, repair or otherwise enhance the performance of the lumen. Percutaneous transluminal angioplasty (PTA), for example, is a procedure used to increase the blood flow through a coronary artery, for example, at a location where the artery is damaged or is susceptible to collapse. The stent, once in place, reinforces that portion of the artery allowing normal blood flow to occur through the artery.
One type of stent which is often used for implantation in arteries and other body lumens is a cylindrical stent which can be radially expanded from a first smaller diameter to a second larger diameter. Such radially expandable stents can be inserted into the artery in the smaller diameter state via the use of a stent delivery device and fed internally through the arterial pathways of the patient until the unexpanded stent is located where desired.
Such radially expandable stents are available in both self-expanding and in balloon-expandable varieties. Once the stent is at the desired location, the stent may be expanded using an appropriate expansion mechanism depending on the type of stent employed.
Self-expanding stents may be formed from a shape memory metallic and/or super-elastic material such as nickel-titanium alloy or nitinol. Such materials have two distinct solid phases. These phases are a high yield strength austenite phase and a lower yield strength martensite phase.
Shape memory characteristics are imparted to the metal by heating the metal at a temperature above which the transformation from the martensite phase to the austenite phase is complete, or the austenite phase is stable, i.e. the Af temperature. The shape of the metal during this heat treatment is the shape which will be remembered.
The heat treated metal is cooled or chilled to a temperature at which the martensite phase is stable, or the metal is transformed from the austenite phase to the martensite phase. Thus, the metal can be selectively transformed between the austenite phase and the martensite phase by altering the temperature of the shape memory metal. This allows stents formed of shape memory metals, for example, to be manipulated such that the stent is in the low yield strength martensite phase when chilled to a temperature below body temperature and to be in the high yield strength austenite phase when the stent is at body.
The metal in the martensite phase is easily plastically deformed to facilitate entry into a patient's body with subsequent heating of the metal in the deformed martensite phase to a temperature above which the martensite phase is stable, therefore transforming the metal the austenite phase wherein the metal reverts to its original shape when unrestrained. If restrained, the metal will remain martensitic until the restraint is removed.
When stress is applied to a shape memory metal which exhibits superelastic characteristics, at a temperature above that at which the austenite phase is stable, i.e. the temperature at which transformation from the martensite phase to the austenite phase is complete, the specimen deforms elastically until it reaches a particular stress level where the shape memory metal then undergoes a stress-induced phase transformation from the austenite phase to the martensite phase
Additionally, when such shape memory alloys are stressed beyond their yield strength while in the martensite phase, not to exceed certain maximum amounts of strain, the alloy has a “memory” of its shape before its yield strength in the martensite phase was exceeded so that when the alloy is heated and transformed into its austenite phase it returns to the shape it exhibited before it was plastically deformed in the martensite phase.
For radially expandable surgical stents, this shape memory has been used to collapse the stent to a small diameter when in its martensite phase where it can be deformed relatively easy, and then heat the stent up to body temperature and transform the stent into its austenite phase where it radially expands back to its original expanded diameter and exhibits a desired strength and size for supporting walls of the body lumen in which it is implanted. As the temperature is increased to its austenite condition, it reverts to its original shape using relatively high force.
Thus, the relatively high yield strength of the shape memory alloy stent in its austenite phase provides beneficial characteristics for supporting the body lumen while the martensite phase for the shape memory alloy stent is utilized to allow the stent to be easily radially contracted and deformed during implantation of the stent.
These shape memory alloys, however, are not highly radiopaque. For accurate positioning in a body lumen during a clinical procedure, it is desirable to be able to visualize the stent using imaging techniques such as fluoroscopy, for example.
One method of achieving radiopacity is to attach markers to the stent. For example, see U.S. Pat. No. 6,863,685, the entire content of which is incorporated by reference herein. Some attachment methods can lead to undesirable stress on the stent structure.
The information described above is not intended to constitute an admission that such information referred to herein is “prior art” with respect to this invention.
All US patents and applications and all other published documents mentioned anywhere in this application are incorporated herein by reference in their entirety.
Without limiting the scope of the invention a brief summary of some of the claimed embodiments of the invention is set forth below. Additional details of the summarized embodiments of the invention and/or additional embodiments of the invention may be found in the Detailed Description of the Invention below.
A brief abstract of the technical disclosure in the specification is provided as well only for the purposes of complying with 37 C.F.R. 1.72. The abstract is not intended to be used for interpreting the scope of the claims.